The present invention relates to a method of treating starch with organosiloxane polymers and to the treated starch. The present invention further relates to the use of said treated starch in a process for sizing substrates, such as paper or textiles and to the sized substrate.
Starch is used in a wide variety of applications. It is often used as a glue, a binder, or a filler for polymeric compositions such as plastic or rubber. One particularly important use of starch is as sizing.
In the sizing process, starch is used to level surfaces, thus rendering the surfaces smoother. For example, starch sizing is applied to paper to make it smoother, glossier, and more receptive to printing inks. Textiles are sized with starch to provide better body, stiffness, and smoothness.
It is often desirable to modify starch chemically. For example, starch is modified to produce less viscous aqueous dispersions of the starch, or the starch is modified to be more water repellent in the dry state.
Increased water repellency is a highly desirable property in starch used as glue or binder material. Starch of greater water repellency is especially desirable for sizing paper to be used in an offset printing process.
Offset printing requires that the paper be wetted with water except where it is desired to print. Rewetting occurs for each color printed. After a series of wettings, untreated starch sizing may be solubilized, allowing the underlying paper to swell and weaken. The swollen paper breaks more easily and cockles upon drying.
A number of attempts to modify starch with various silicones have been made.
Griffin, in U.S. Pat. Nos. 4,021,388 and 4,125,495, discloses the treatment of starch with sodium alkylsiliconate in water, as well as the treatment of starch, in the dry state, with hydrogen polydimethylsiloxane. Starches treated by each of these two methods were used as fillers for biodegradable plastic compositions. Silicone treatment of the starch improved the strength of the plastic compositions.
Musolf, in Canadian Pat. No. 921,204, discloses the incorporation of a solid siloxane into an aqueous slurry of starch. Textiles subsequently sized with the starch were more soil repellent.
British Pat. No. 891,582, discloses the use of an alkali metal siliconate to treat starch. The alkali metal siliconate was added to starch in cold water. Patentees in this case found that methylhydrogensiloxane and dimethylsiloxane compositions were not suitable for treating starch in a room temperature aqueous dispersion because the treated starch produced an uneven, fish-eyed, surface when applied to paper as sizing.
Kokoszka and Yates, in U.S. Pat. No. 3,462,384, disclose the incorporation of a polydimethylsiloxane emulsion in an aerosol starch formulation, to improve the spray pattern and diminish the lateral force necessary to iron clothes sprayed with the aerosol starch formulation. The silicone emulsion in this case was added to an aqueous starch dispersion cooled to room temperature.
Simonides, in U.S. Pat. No. 3,409,136, discloses the addition of an aqueous methylsilicone oil suspension to a cool aqueous starch dispersion, for the purpose of rendering an adhesive, based on the treated starch, more water repellent.
Pepe, in U.S. Pat. No. 3,419,422, discloses isocyanate functional polydiorganosiloxanes, which can provide a water repellent paper size either per se, or when mixed with a starch dispersion.
None of the above patents disclose the present invention, which comprises contacting starch and a silicone, in an aqueous dispersion, at a temperature of from 60.degree. C. to 98.degree. C., for a period of time at least 60 seconds in duration. The addition of silicones to dry starch or the addition of silicones to a room temperature aqueous dispersion of starch, is less effective than the method of the present invention.